Highly abrasive grit materials are well known in the art and are commonly used as blast media in sandblasting operations. Typical applications for highly abrasive grit blast media include cleaning building exteriors and removing surface coatings such as surface oxides on castings. In applications such as these, minor surface damage caused by the blasting operation is not of concern and abrasive grit which is relatively aggressive in removing surface coatings may be used. However, other applications exist in which the surface damage which results from the use of traditional highly abrasive grit blast media is unacceptable.
One example of a particularly sensitive application for abrasive grit materials is in the military and commercial aircraft industry. The removal of paint from exterior aircraft surfaces is periodically required for cosmetic purposes and to allow technicians to inspect the underlying metal and other surfaces for corrosion. However, aircraft surfaces clad with aluminum or made of composite materials are particularly sensitive to damage caused by conventional blast media.
Aircraft surfaces are typically clad with pure aluminum, which is softer than the underlying aluminum alloy the cladding is designed to protect. The soft layer of aluminum cladding is particularly susceptible to damage and erosion by traditional blast stripping operations, which can leave a very rough and unacceptable surface finish. Such damage to the aluminum cladding can result in premature corrosion that could necessitate the replacement of whole sections of an aircraft's skin. Furthermore, methods used for stripping aluminum clad structures may not be acceptable for structures made of composite materials such as fiberglass, graphite and aramid (Kevlar®), which are even more sensitive than aluminum to damage from blast media.
Prior art blast media used in aircraft applications include synthetic hard abrasive grit materials composed of thermosetting resins such as epoxy resin, urea resin, unsaturated polyester resin, alkyd resin, or harder resins such as polystyrene, polycarbonate or acrylic. Although less aggressive than conventional blast media, plastic blast media are sufficiently aggressive when used in paint stripping applications that skilled handling by trained personnel is required in order to avoid permanent damage to aircraft skins. Moreover, even skilled use of such blast media can result in reduced fatigue life, increased crack propagation and high surface roughness to the extent that such media is unacceptable for repeated use on aluminum clad or composite structures.
Gentler methods of paint removal involve the use of chemical solvents such as methylene chloride or phenol. However, health dangers and stringent environmental laws associated with the use of such volatile organic chemicals in paint stripping operations has made the use of such solvents impractical and undesirable. In some cases, companies have postponed aircraft exterior surface maintenance programs rather than attempt to deal with the regulations and costs associated with chemical stripping.
A series of related patents, owned by the assignee of the present invention, describe entirely new types of less destructive abrasive grit material. U.S. Pat. Nos. 5,066,335; 5,360,903; 5,367,068; 5,780,619; and 6,159,257 describe glass-like polysaccharide abrasive grit particles made from conventional starches, such as wheat and corn starches. These abrasive grit particles comprise a particulate polysaccharide glass-like solid in which moisture is occluded within the interstices of a matrix of polysaccharide molecules. The polysaccharide molecules may be partially cross-linked to increase the complexity of the matrix of polysaccharide molecules within the glass-like polysaccharide grit particles.
The glass-like polysaccharide abrasive grit compositions described in these patents enjoy several advantages over the prior art abrasive grit. Polysaccharide abrasive grit particles are less aggressive than plastic blast media, thereby avoiding the damage to aluminum clad structures caused by traditional blast stripping methods. As a result, use of polysaccharide abrasive grit is more forgiving and requires less skill in handling than plastic blast media. Use of polysaccharide abrasive grit allows aluminum clad aircraft skins to be subjected to repeated paint-and-strip operations. This is in contrast to plastic blast media which some aircraft manufacturers limit in paint stripping operations to only once in the lifetime of an aircraft, due to the potential damage to the aircraft's surface. Furthermore, polysaccharide abrasive grit particles are acceptable for use with both aluminum and composite materials, allowing for complete nose-to-tail paint stripping operations of aircraft. Finally, abrasive grit particles made of polysaccharides such as wheat and corn starch are non-toxic and biodegradable, reducing the health risks and disposal problems often associated with paint stripping operations. These advantages over other methods have allowed the glass-like polysaccharide starch grit particles described in these patents to gain wide acceptance in the industry for removing coatings from aircraft.
Additionally, the polysaccharide grit particles described in the aforementioned U.S. Pat. No. 6,159,257 have an additional advantage in that they maintain their integrity particularly well when placed in contact with liquid water and retain their usability as blast media particularly well after they are dried. In addition, the abrasive blast media described in this patent facilitate detection and clean up of particles that infiltrate the joints and seams present in blast-treated surfaces.
Certain applications relating to cleaning, polishing, and abrading delicate surfaces found in aircraft and elsewhere call for a blast media which is particularly gentle-acting. Such a gentle-acting blast media should produce cleaning, polishing, and abrading results as rapidly and efficiently as the polysaccharide grit media described above. If the previously patented polysaccharide grit media could be modified to achieve such a result, the art would be substantially advanced.
Additionally, the effective removal of certain elastomeric coatings from delicate substrates without damage to the substrate presents a particular challenge. Examples of such elastomeric coatings include, from the aerospace industry, the following: high strength aircraft integral fuel tank sealant (polysulfide); high strength, high temperature aerospace grade acetoxy paste sealants (silicone rubber); rapid cure primeness aircraft integral fuel tank sealant (alcohol/solvent-based); low density, high temperature aircraft integral fuel tank sealant (polysulfide); vertical surface sealant and bonding (silicone rubber); fuel and solvent resistant paste adhesive sealant (fluorosilicone); and medium density ablator/insulator aircraft thermal protection coating (filled elastomeric silicone (MA-25)). Such elastomeric coatings have Shore A hardness of about 40 to 60. A gentle-acting abrasive particle that would remove such materials with a Shore A hardness of about 40 to 60 without damaging delicate substrates would be a welcome addition to the armamentarium of abrasive grit products.
Finally, polysaccharide grit would be usable in cleaning and abrading interior surfaces of enclosed vessels such as the inner walls of the fuel tanks, for removing elastomeric sealants while leaving the underlying protective primer coat intact, it could be insured that the grit particles will be easily and completely removed from the vessels when the cleaning and abrading is complete. Thus, if the polysaccharide grit particles could be modified to float out on water or other solvents used to rinse such vessels, another substantial advance in the art would be at hand.